hrm, tips of ease of killing:
Force field is probably the most powerful spell in the game. use it on enemy spell casters, or on whomever is just about to die.
healing spells are also very potent, if you have both magic users in your party healing a tank it is very unlikely for you to die.

tactics: if you are facing too many foes at once, try to use the narrowness of doors to limit the number of attackers or to force their ranged attackers to move back into line of sight where you might have planted traps.

as far as powerful builds go, i find that a magic user gets the most dps at lower levels from chain casting the elemental single target spells, so dip into each of the elements to get atleast 3 spells to chain. This you can do effectively until you reach higher levels where you might want to instead chain cast area effect spells while leaving either a repulsion ward or a force field protected character to keep them inside the area of effect.

hmm, tactics for the non magic wielding party though....
thats really difficult i would say since it seems as though mages are the only class with any real versatility to them tactically. Sure warriors and rogues do have some small variance, but no where near the range of a mage.
So, i would say a 2 handed warrior is better than a duel wielding warrior in any situation where you might be getting stunned or bull rushed a lot; ie against a lot of archers or an ogre. A shield warrior is only good if you have someone else doing the dps because they basically have no dps.
a Rogue is very potent if always used from behind but quite pathetic in any other manner (which is of course to be expected) Aside from that - dexterity enough to duel wield with momentum + all the rest into cunning + the stunning moves + assassin stuff equals dead stuff really fast.